1. Field of the Invention
The present invention relates generally to miniature power sources and, more particularly to miniature two-cycle combustion engines combined with linear electrical generators.
2. Background Information
There are many applications (for example, in tools, toys, bicycles, recreational equipment, micro surveillance airplanes, laptop computers, and other electronic equipment) where small, light-weight, high power density and energy capacity, power sources are needed.
At the present time such applications are being filled with items from a variety of battery technologies. Lithium batteries, for example, are being used as long-lived power sources especially for computer memories. Nickel-cadmium and nickel-metal batteries are used for power tools. Other electrochemical systems are used for other applications. All batteries, however, have poor power density and energy density (less than 300 watt-hours per kilogram). These parameters have limited the use of such batteries where power and weight are factors.
Fuel cells are another possible candidate for such uses, but despite their promise they have proven to be technologically difficult to develop for these applications. Also miniature gas turbines and the like are possibly being developed for high power miniature power source applications.
Yet another approach is to reduce the size of a standard two or four stroke engine as the driver of an electrical generator. But this approach entails a number of problems. One set of problems of directly reducing the size of such engines involves the small linkages, articulated joints, valves, etc. that would require precise machining and close tolerances. Although such engine/generators have been built and operated, as a standard internal combustion engine is reduced in size, such mechanics become expensive, the power density and energy capacity are reduced, and the mechanical linkages have shorter lives (specially if attempts are made to maintain power density by incurring higher stresses and loads). While in situations where shaft power is required these problems might be tolerable (model aircraft engines, for instance), for electrical power the need for a separate generator and the above discussed problems make this approach unattractive.
An M.I.T. Master""s thesis, submitted in June of 1996, by Matthew D. Greenman, entitled Design and Construction of a Miniature Internal Combustion Engine, describes a miniature internal combustion engine generator design. This thesis is incorporated by reference herein. This thesis describes an approach to a design that is meant to take advantage of the high energy content of hydrocarbon fuels (in the range of 40,000 J/g) while eliminating many moving parts found in a standard internal combustion engine. The engine is a two-cycle piston engine with a cylinder bore and a stroke of a few millimeters. The piston is attached to a mechanical spring and an induction coil that is positioned to reciprocate in the air gap of a permanent magnet. Electrical power is delivered from the induction coil to the external load. The engine operates at or near the resonant frequency of the active-mass-spring (active mass=masses of coil+standoff+piston+piston rod+portion of the spring) system. As mentioned above, there are no valves or articulated mechanical linkages. The design presents an approach to many issues but does not fully examine practical solutions.
One troublesome area in the thesis design is the double helix spring made by forming wire and then brazing or welding the ends to separate end pieces. This spring design suffers from tolerance problems and does not restrict transverse movement enough for a practical design. In addition, it has the practical difficulty of attaching the end pieces while maintaining perfect symmetry of the two helices.
There is a continuing need for and it is an object of the present invention to provide a high power density, high-energy capacity, miniature engine generator.
There is a need and it is another object of the present invention to reduce the number of parts and to control better the mechanical parameters and to reduce the number of assembly steps to fabricate a miniature engine generator.
The objects set forth above, as well as further and other objects and advantages of the present invention, are achieved by the embodiments of the invention described hereinbelow.
A miniature internal combustion engine generator invention is constructed with a cylinder head, a fuel-air-mixture inlet port, an exhaust port, and an axial groove in the cylinder wall that provides a transfer port to allow the engine to run in a two-cycle mode. A piston is constructed and arranged to travel reciprocally within the cylinder, and a rod extends axially from the piston. In another aspect of the invention, the operation of the motor may be of the four-cycle variety.
A spring formed, preferably machined, from one piece of metal stock, preferably titanium or an alloy thereof, is formed as a double helix. However, a triple helix or greater can be used to advantage. One end of the spring is fixed in relation to the cylinder head, and the rod is attached to the other end of the spring.
At the end of the spring and/or rod, distal from the piston, an electric induction coil is fixed and located so that the coil reciprocates within the air gap of a permanent magnet following the piston as the motor operates. Alternatively, the coil can be fixed and the magnet moved.
A housing is attached to the cylinder head and extends over the spring, coil and permanent magnet. The housing may be an extension of the cylinder head assembly, in a preferred embodiment. The ends of the coil electrical wire are made available at terminals where a load made be attached and electrical power drawn.
In a preferred embodiment, one electrical lead from the coil is in electrical connection to the helix spring and the housing to which terminal may be mounted. The other lead is insulated from the spring and housing and is arranged via a weak spring to an insulated terminal. The weak spring allows the lead to move without incurring fatigue problems in the lead or spring and to provide a minimal mechanical load on the system. The electrical load is attached between the terminals. The spring is formed or made from a single piece of stock, preferably titanium or an alloy thereof, with integral end fittings. The end fittings form the mechanism whereby the spring is fixed to the cylinder head and extension and/or housing. Preferably, the end fitting is a shoulder or a threaded connection on the spring and there is a mating shelf or threaded connection on the housing or the cylinder. In a preferred embodiment, there is a mating shelf built into the joint between the housing and the cylinder and an external clamp is used to secure the spring, housing and cylinder head together. But, other means of fixing the spring to the housing/cylinder head are known and can be used to advantage in the present invention.
The spring may be attached in one embodiment at the cylinder head with the piston rod attached to the far end of the spring. But, in another embodiment, the spring is fixed to the housing distal from the cylinder head and the rod is attached to the near end of the spring. In this second embodiment the rod is farther extended through the center of the spring to attach to and thereby axially move the induction coil.
In other preferred embodiments, the electrical generator coil and magnet may be positioned between the piston and the helix spring.
For a better understanding of the present invention together with other a further objects thereof, reference is made to the accompanying drawings and detailed descriptions, and its scope will be pointed out in the appended claims.